Wye-Wye

[ppsh]

Have a customer with a 480/277v service that feeds a 75kva 480D-4160D transformer. The 50 year old transformer has failed, currently running generators to get them going. Customer owned generators, so they don't have too much of an issue running these for a while.

125a 3w w/ ground feed to the transformer location. 4160 transformer feeds 2 outbuildings and pump sheds ~2 miles away. Two of the stepdown transformers are single phase 4160-120/240, connected L-L. Pump shed has a 4160d-480/277wye stepdown. They have a presumably working 75kva 4160/2400wye-480/277wye transformer on hand.

Is this wye-wye transformer useable in this situation? Do I need to provide a neutral to the transformer? RMC 500ft from service to transformer location, so doubtful to be able to get a neutral to it. But is one even needed if there is nothing connected L-N on the 4160 side? I know to leave them floating if you are supplying a wye-delta, but have not run into a wye-wye before. Can have a matching transformer in a week or so, but curious if this will work for now.

 

[winnie]

Wye-wye transformers are known to have unstable L-N voltage on the secondary because there is no path for the reflected unbalanced L-N current on the primary side (unless you connect the primary neutral).

But with no secondary L-N loads, I think the system would work...except possibly for grounding. I'm not sure what the 4160V L-G voltage would do, and unstable L-G voltage puts the insulation of the 4160V system at risk.

I'm not a medium voltage guy, just guessing on theory here

 

[jim dungar]

When you run a Wye connection backwards you should all but never connect the source neutral. Let it float so unbalanced loads can be accommodate easily.
This is similar to how we ignore the Wye point of motor windings.

The secondary side probably should be grounded like the existing system already is.

 

[electrofelon]

When you run a Wye connection backwards you should all but never connect the source neutral. Let it float so unbalanced loads can be accommodate easily.
This is similar to how we ignore the Wye point of motor windings.

Jim are you sure about that? I agree it is important to leave the wye point floating on a wye-delta, but this is a wye-wye and I thought for the reasons Jon mentioned you need the primary neutral? FWIW I have never seen the utility leave a wye primary floating.

 

[winnie]

A wye:delta transformer derives a low impedance neutral. This is why these transformers are desirable for creating a stable secondary neutral, and why you almost never connect a primary neutral.

Imagine you have an existing wye system with a grounded neutral. If you connect all 4 wires to the primary of a wye (primary): delta (secondary) transformer the derived neutral on the transformer will be placed in parallel with the system neutral, and unless everything is perfectly balanced you can see large circulating currents as the two neutrals fight it out.

Utility companies might connect to the primary neutral of a wye:delta (or wye:delta:wye) transformer as part of their 'effective grounding' needs, where they are specifically taking advantage of that low impedance neutral to maintain stable L-G voltages during fault conditions.

A wye:wye transformer does _not_ have a low impedance neutral.

Hunt down the GE document 'The Whys of the Wyes'

-Jonathan

 

[jim dungar]

Jim are you sure about that? I agree it is important to leave the wye point floating on a wye-delta, but this is a wye-wye and I thought for the reasons Jon mentioned you need the primary neutral? FWIW I have never seen the utility leave a wye primary floating.

First, you are not a utility system do not try to act like one.

Second, fixing the source side into a Wye will typically cause more problems than it solves. See my first point.

 

[electrofelon]

First, you are not a utility system do not try to act like one.

 

[wwhitney]

When you run a Wye connection backwards you should all but never connect the source neutral. Let it float so unbalanced loads can be accommodate easily.
This is similar to how we ignore the Wye point of motor windings.

Want to check my understanding here--the above applies to a single wye-wye transformer with a figure 8 core, which imposes a restriction on the magnetic side that effectively imposes the restriction that the applied voltages must sum to zero.

Whereas if you use 3 single phase transformers in a wye-wye configuration, or a wye-wye transformer with a core with an additional magnetic pathway (e.g. 5 leg), no such restriction is imposed. In which case if you connect the primary side neutral, any imbalance on the primary side just gets reflected on the secondary side, and vice versa.

Is that correct?

Cheers, Wayne

 

[jim dungar]

Whereas if you use 3 single phase transformers in a wye-wye configuration...

This is one of the things of how a utility does things differently than us NEC people.

The large utility here has required triplex core construction for their padmount transformers for at least 40 years now.

 

[ggunn]

A client of mine consulted me about a transformer to connect some 480V inverters to a 208V service. I specified a delta 208V primary and a 480/277V wye xfmr , but they bought and installed a 208/120V wye primary, 480V delta secondary xfmr. I talked to the inverter company (SolarEdge), and luckily for my client, the inverters will run on 480V delta so he does not have to replace the xfmr. Do you guys recommend that he disconnect the neutral from the 208V side of the xfmr?

 

[jim dungar]

Do you guys recommend that he disconnect the neutral from the 208V side of the xfmr?

Absolutely.

 

[electrofelon]

A client of mine consulted me about a transformer to connect some 480V inverters to a 208V service. I specified a delta 208V primary and a 480/277V wye xfmr , but they bought and installed a 208/120V wye primary, 480V delta secondary xfmr. I talked to the inverter company (SolarEdge), and luckily for my client, the inverters will run on 480V delta so he does not have to replace the xfmr. Do you guys recommend that he disconnect the neutral from the 208V side of the xfmr?

Agree with Jim, absolutely do not land the 208 neutral, let it float.

Just curious can you corner ground it or do you need ground detectors?

 

[ggunn]

Agree with Jim, absolutely do not land the 208 neutral, let it float.

Just curious can you corner ground it or do you need ground detectors?

The inverters in question can run on an ungrounded delta but not on a corner grounded service.

 

[electrofelon]

The inverters in question can run on an ungrounded delta but not on a corner grounded service.

So again, just curious, as I have never had to provide them, what are you going to do for ground detectors?

 

[ggunn]

So again, just curious, as I have never had to provide them, what are you going to do for ground detectors?

I have never dealt with PV inverters on an ungrounded delta before, but the inverter tech support engineer I spoke with did not mention ground detectors when he told me that the inverters would connect and run.

 

[wwhitney]

I have never dealt with PV inverters on an ungrounded delta before, but the inverter tech support engineer I spoke with did not mention ground detectors when he told me that the inverters would connect and run.

OK, but we are telling you that NEC 250.21(B) will require ground detectors for your ungrounded 3-wire 480V delta system, so you'll need to figure that out.

Cheers, Wayne

 

[electrofelon]

I have never dealt with PV inverters on an ungrounded delta before, but the inverter tech support engineer I spoke with did not mention ground detectors when he told me that the inverters would connect and run.

OK, but we are telling you that NEC 250.21(B) will require ground detectors for your ungrounded 3-wire 480V delta system, so you'll need to figure that out.

Cheers, Wayne

Might be better to just corner ground it? If an inverter can run on an ungrounded system, does that mean it can run on a grounded system? My first thought is yes but I would confirm.

 

[jim dungar]

Might be better to just corner ground it? If an inverter can run on an ungrounded system, does that mean it can run on a grounded system? My first thought is yes but I would confirm.

GF Detectors are easy. All it takes is three 480V rated pilot lights connected in a Wye configuration with the center point grounded through a normally closed pushbutton for testing.

There are lots of schematics on-line.

 

[don_resqcapt19]

GF Detectors are easy. All it takes is three 480V rated pilot lights connected in a Wye configuration with the center point grounded through a normally closed pushbutton for testing.

There are lots of schematics on-line.

The first ones I saw were three sets of two 220 volt lamps connected in series and then each set connected in a wye with the center grounded.

 

[electrofelon]

Sure it's easy, but perhaps not the best solution in some cases.